Alloy and method of welding structures including this alloy

ABSTRACT

An iron-nickel-chromium alloy with which crack-free sound welds are produced for fuel elements and tube-to-tube sheet assemblies having low carbon (0.05 max) and up to 0.40 each of titanium and aluminum, the Ti being eight to 10 times the carbon and the aluminum four to six times the carbon. The Ti and/or Al suppress the formation of massive Cr23C6 molecules. Also a method of welding iron-nickel-chromium alloy in which Ti and/or Al are present to prevent cracks from forming.

United States Patent 11 1 Boyko et al.

[ Oct. 2, 1973 ALLOY AND METHOD OF WELDING STRUCTURES INCLUDING THIS ALLOY [75] Inventors: Eugene S. Boyko, Monroeville; Clark M. Owens, Irwin, both of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Jan. 17, 1967 {21] Appl. No.: 611,533

[52] U.S. Cl 75/122, 75/124, 75/128 {51] Int. Cl C22c 39/02, C22c 39/20 [58] Field of Search 75/122, 128, 134,

[56] References Cited UNITED STATES PATENTS 2,054,405 9/1936 Becket et al 75/128 2,054,770 9/1936 2,240,672 5/1941 3,159,479 12/1964 3,177,577 4 1905 Fujimura et a1. 75 12& x

FOREIGN PATENTS OR APPLICATIONS 114,424 9/1947 Australia 75/171 OTHER PUBLICATIONS Properties of Some Metals and Alloys, The International Nickel Co., Inc., New York, page 21, 1962.

Primary Examiner-Charles N. Lovell Att0rney--A. T. Stratton and Z. L. Dermer [57] ABSTRACT Also a method of welding iron-nickel-chromium alloy in which Ti and/or A1 are present to prevent cracks from forming.

1 Claim, 28 Drawing Figures mmggncr 2191a SHEET RBID CA Es GRAIN BOUNDARIES G 6 m A m Nll NW AT TYPICAL MICROSTRUCTURE 0F ANNEALED INCOLOY 800 ,N) PRECIPITATES IN ANNEALED INCOLOY 800 INVENTORS Eugene S. Boyko and Clark ATTORNEY Pmmmncr 2x975.

WET 9311f 13 TLC, Tj.N, Cr C PRECIPITATES IN ANNEALED INCOLOY 800 Q 0 PRECIP ITATES IN ANNEALED INCOLOY 800 s mm mm EB v m5 8 n e g u E S n e and Clark mam-mm 2 1 3,782,913

SHEET 6; HF 13 FIGS ELECTRON DIFFRACTION PATTERN OF A DENTRITIC PRECIPITATE IDENTIFIED AS Cr C INVENTORS Eugene S. Boyko one Glor' M wens PATENTEnnm 2:973 3,762,913

saw usnr 13 m m F WELD PUDDLE CRACKING IN ANNEALED INCOLOY 800 INVENTORS Eugene S. Boyko and Clark 0 ens RNEY Cr C DENDRITE IN AN EARLY STAGE OF DEVELOPMENT EXTRACT FROM WELD OF ANNEALED INCOLOY 800 PATENJEUQBT 21915 SHEET 08 HF 13 TYPICAL ELECTRON MICROSTRUCTURE OF INCOLOY s00 PLATE GRAIN BOUNDRY PREClPlTATES SHOWING i KA m g m .8 BR A RC U Mm Al 3 W A G R A KL ME m N R A B INVENTORS Eugene S. Boyko' onq Clu Z:eus

A O NEY PATENTEDHBT 21m DIAMETER sum 0? HF 13 TYPICAL ELECTRON MICROSTRUCTURE OF B-O ALLOY BAR STOCK TYPICAL ELECTRON MICROSTRUCTURE OF DIAMETER B-O ALLOY BAR STOCK INVENTORS Eugene S. Boyko and Clark I ATTORNEY PAIENTEHHBT 2 3,762,913

sum new 13 0 UK T V. mm V MS mm 0 gl E U6 5 mm m w. m e n 6 m m mm P F E R L 4 w ww mfi L n w D mm Hm C MO 0 8 ELECTRON MICROSTRUCTURE OF 0.0|O WALL ELECTRON TRANSMISSION OF 5 NOTE PRESENCE OF DISLOCATION ELECTRON MICROSTRUCTURE OF 0.0IO' WALL SHEET 09 [1F 13 i 82 FILLER WIRE INVENTORS Eugene S and Clark Bo y k0 ens AT RNEY PAIENIEBnm 2% B-O ALLOY WELDED WITH N PATENIEDHCT 2 3,762,913

S8ET 10 8F 13 B-O ALLOY WELDED WITH INCO I38 BARE FILLER WIRE FIG.2|.

B-O ALLOY WELDED WITH 330 STAINLESS FILLER WlR INVENTORS Eugene S. Boyko and Clark PATENIEDBBT "2 3.762.913

Sam 11 0F 13 in I WW F I G. 22.

B-O ALLOY T0 304 SS WELDED WITH Ni 82 FILLER B-O ALLOY TO 304 SS WELDED WITH INCO I38 BARE FILLER WIRE INVENTORS Eugene S. Boyko and Clark ens ATTORNEY Pmmmmm 35,762,913

sum 12 or 1 FIG.24.

B-O ALLOY TO 304 SS WELDED WITH 330 SS FILLER WIRE FIG.25-

B-O ALLOY TO 3|6 SS WELDED WITH Ni 82 FILLER WIRE INVENTORS Eugene S. Boyko and Clark ens BY ATT RNEY PATENTED BUT 2 sum 13 er 1s B-O ALLOY TO 3l6 SS WELDED WITH INCO I38 FILLER WIIRE B-O ALLOY TO 316 SS WELDED WITH 330 SS'FILLER WIRE ALLOY AND METHOD OF WELDING STRUCTURES INCLUDING THIS ALLOY BACKGROUND OF THE INVENTION This invention relates to the ferrousalloy art and has particularly relationship to the fabrication by welding of structures including parts of iron-nickel'chromium alloys.

An iron-nickel-chromium alloy, sold by International Nickel Company under the name Incoloy 800, and having approximately 45 percent iron, 32 percent nickel and 21 percent chromium has been found to have advantageous properties for use in power-generation apparatus in which the heat-exchange fluid is a corrosive material such as liquid sodium. Typically, this alloy has the following composition:

TABLE I C Mn Fe S Si Cu 0.10 1.50 E31. 0.03 1.00 max. max. max. max.

Ni Cr 30-35 19-23 This alloy has favorable high-temperature mechanical properties, particularly high tensile-stress resistance and high ductility at temperatures of l,l50 F., and it also has corrosion resistance at the same high temperatures to liquid sodium and steam. Attempts have been made to use this alloy for cladding for fuel elements and for tube-to-plate or tube-to-tube sheet assemblies of heat exchangers. Such use requires that the cladding or assemblies be sealed by sound pressure-tight welds which must necessarily be crack-free. This requirement is particularly stringent as to assemblies in the light of the 20 or 30 year design life which is demanded of heat exchangers. The attempts have failed because it has not been possible to produce crack-free welds.

It is an object of this invention to overcome the above-described disability of the iron-nickeLchromium alloy and to provide an alloy of the iron-nickelchromium type with which crack-free welds shall be readily producible and to provide cladding for power generators and tube-to-plate assemblies using this alloy.

It is also an object of this invention to provide a novel method for producing crack-free welds with ironnickel-chromium alloys.

This invention arises from the discovery that the cracks in the welds are caused by the presence of chromiumcarbides, particularly massive molecules of Cr,,C, in the parts used and their formation, during the welding operation. These chromium carbides precipitate in-grain boundaries in the sheet and tubes used to make the structures and during the welding operation these carbides and others, produced under the heat of the welding arc, cause the cracks to form. In accordance with this invention, the formation of cracks is suppressed by including in the alloy adequate material having an affinity for carbon which may form carbides or nitrides dispersed throughout the matrix. An important such material is titanium; aluminum also serves as such material; and both Ti and Al may be used.

Another aspect of this invention is to maintain the carbon at a low magnitude. Specifically, an iron-nickelchromium alloy is provided in which the carbon is maintained at less than about 0.05 percent and the titanium is maintained at between eight and 10 times the carbon content. The alloy according to this invention is herein called B-0 Alloy.

A recommended composition in accordance with this invention of this B-0 Alloy, with which are produced crack-free welded structures having high resistance to tensile stress and high ductility at high temperatures of about l,l50 F. and high corrosion resistance to steam and liquid sodium at these high temperatures, is as follows:

TABLE II c Mn s I Si Ni .05 1.50 .030 .03 L0 30-35 max. max. max. max. max.

c: Fe Cu Ti A1 C0 19-23 Bal. .75 8-IOC 4 -8C .10

max. .40 .40 max.

max. max.

It is also recommended that B-0 Alloy material for these structures should be solution annealed at 2l50F.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a view in longitudinal section of a fuel element in which the alloy according to this invention is included and which is made in the practice of the method of this invention;

FIG. 2 is a view in perspective of a tube-to'tube sheet assembly in which the alloy according to this invention is included and in the making of which the method of the invention is practiced;

FIGS. 3, constitutes a plan view and a view in side elevation of a typical sheet of a tube-to-tube sheet assembly as shown in FIG. 2 prepared for welding, and is included for the purpose of aiding those skilled in the art in practicing this invention and not with any intention of in any way limiting this invention;

FIG. 4 is a photomicrograph (at magnification 500K) of a specimen of the prior art iron-nickel-chromium alloy solution annealed at 2,100 F.;

FIG. 5 is a similar photomicrograph of a specimen of this alloy;

FIG. 6 is an electron micrograph (at magnification 5,900X) of a specimen of this alloy; 4

FIG. 1 is another electron micrograph (at magnification 12,500X) of a specimen of this alloy;

FIG. 8 is another electron micrograph (at magnification 23,000X) of a specimen of this alloy;

FIG. 9 is an electron diffraction pattern of a precipitate in this alloy;

FIG. 10 is a photornicrograph of a specimen of the weld zone of a fusion weld produced with the alloy from which FIGS. 1 through 9 were derived; I

FIG. 10A is an electron micrograph (at magnification 1,500X) of a specimen of this weld zone;

FIG. 11 is an electron micrograph (at magnification 23,000X) of a specimen of plate of another prior an alloy showing precipitate concentrated at grain boundaries;

FIG. 12 is a photomicrograph (at magnification 500X) of a specimen of bar stock of alloy according to this invention (B-0 Alloy);

FIGS. 13, 14 and 15 are electron micrographs (at magnifications 6,000X, 13,600X and 24,000X respectively) of this alloy;

FIGS. 16 and 17 are electron micrographs (at magnifications 6,000X and 13,600X respectively) of a specimen of tubing of the iron-nickel-chromium alloy to which were welded lugs of the bar stock whose micrographs are shown in FIGS. 12 through 15 and the welds were found to be crack-free;

FIG. 18 is a photomicrograph (at magnification 500X) of a specimen of this tubing; and

FIGS. 19 through 27 are photomicrographs (at magnification 50X) of welds produced in joining by TIG welding, with fillers of different compositions, parts of iron-nickel-chromium alloy at least one of which was composed of the alloy according to this invention (B-0 Alloy).

DESCRIPTION OF PREFERRED EMBODIMENTS The alloy of this invention is used in, and the method is practiced in the making of, fuel elements 51 as shown in FIG. 1. An element 51 includes fuel pellets 53 contained in cladding. The cladding includes a shell 55 of 13-0 Alloy which is sealed by plugs 57 of 8-0 Alloy by circumferential welds 59. Insulating spacers 61 are interposed between the fuel pellets 53 and the plugs 57.

The fuel pellets 53 generate intense heat which is transferred to a heat exchange fluid (not shown), typically sodium, in which the fuel element 51 is immersed. Thee cladding typically has a thickness of 0.010 inches to 0.015 inches. The plugs are welded to the shell by fusion welding with a non-consumable electrode in a shield or atmosphere of inert gas (TIG welding) which may be argon or helium. Welding-grade pure gas is used (99.999 percent) purity. Typically, the process taught in US. Pat. No. 3,183,066 Lessman et al. dated May 11, 1963 and in application Ser. No. 603,389 filed Dec. 20, 1966 to Donald R. McClintock continuation of application Ser. No. 275,884, filed Apr. 26, 1963 now abandoned. To prevent the sodium from penetrating into the element, it is necessary that the material of whcih the cladding 55-57 is composed shall not be corroded by the sodium at high temperatures and that the welds be crack-free. High tensile strength and ductility are also demanded. Cladding composed of the B-0 Alloy meets these conditions.

FIG. 2 shows a tube-to-tube sheet assembly 71 including sheet 73 and tubes 75 welded pressure-tight into the sheet 73 by circumferential welds 77. The sheet 73 is composed of 8-0 Alloy and may also be composed of AISI-304 stainless steel. The tubes 75 are composed of 13-0 Alloy. Satisfactory welding is carried out by TIG fusion welding with the shoulder of a trepanned groove (81 FIG. 3) providing the filler material. For this welding, a groove 81 is trepanned around the hole 83 into which a tube 75 is to be sealed. Typical dimensions of a sheet 73 with the hole 83 and groove 81 are presented in FIG. 3.

High temperature corrosion resistant material and crack-free welds are necessary because the tubes 75 carry liquid sodium at a high temperature (1,l50 F.) and are typically immersed in highly corrosive steam at a high temperature. The 3-0 Alloy meets these conditions and in addition has the necessary high tensile strength and ductility at high temperatures.

At the inception of the work from which this invention arose, attempts were made to produce TIG welds 4 for fuel element cladding with Incoloy-800 alloy having the following composition to evaluate the weldability of this alloy:

TABLE III C Mn Fe S Si Cu .04 .87 47.78 0.007 .47 .49 Ni Cr Al Ti 30.42 19.90 0.3 0.1

The specimens for study were TIG welds on flat-plate specimens without filler. Metallographic examination revealed severe cracking in the weld zone occurring generally at weld puddle grain boundaries (FIG. 10). A distinct segregated phase not identifiable by carbon extraction replication techniques was observed within each grain. It was noted that only one type of precipitate was present in the weld puddle metal, i.e., the dendritic structure, Cr, C.,, in an early stage of structure formation (FIG. 10A). The Incoloy-800 alloy proved to be crack sensitive.

A study was made of the alloy. From photomicrographs (not shown) of sections of tubing of the 800 alloy, it was at once noted that sizable precipitates were present near the inner and outer peripheries of the tubing. These precipitates appeared in original specimens and to a lesser extent in specimens solution-annealed at 2,l00 F. The photomicrographs and electron micrographs, FIGS. 4 through 8, were then produced from specimens of the alloy annealed for 15 minutes at 1,800" F. and then for one hour at 2,100 F. The micrographs, FIGS. 4 and 5, showed that carbides and nitrides of titanium were formed and that there were other massive carbides which seemed to be coextensive with the grain boundaries. FIGS. 6, 7, and 8 are extraction replications, FIG. 6 shows TiC and TiN and a chromium carbide precipitate. FIG. 7 shows the precipitate in detail and FIG. 8 shows that it it dendritic. The dendritic precipitate was identified from its electron diffraction pattern as Cr C It was concluded that the crack sensitivity was caused by the chromium carbide and could be eliminated by suppressing the formation of this carbide. This object is accomplished according to this invention by providing an alloy with adequate titanium and/or aluminum to suppress the formation of chromium carbide.

EXAMPLE A This example covers a comparison of Incoloy-800 and B-0 Alloy. The following structures were used in this comparison:

1. one-fourth inch thick plate of Incoloy-800 alloy.

2. five-eighths diameter bar stock of 13-0 Alloy. 3.

0.010 inch thick, 0.400 inch outside diameter tub- TAB LE V C Mn S Si Ni C r Co Ti Al Cu Fe InC010y-800 .05 1.06 .007 .52 30.89 21.32 .10 .025 .45 45.09 B-0 alloy .05 .65 .012 .31 31.86 20.10 .178 .420 .21 46.79

'7 112111 3 was not analyzed chemically. Parts of near" 1 were fusion butt welded by TIG weldings. End plugs were machined from item 2 and fusion welded by TIG welding to the 0.010 inch wall tubing, item 3. Each: weld was sectioned and examined metalographi cally in 0.003 inch increments. Cracks were observed in the welds of item 1; no cracks were observed in the seal welds of items 2 and 3. I

The cracl r sensitivity of 1 and the freedom from crack sensitivity of the welds of item 2 and item 3 were bars were produced of alloys having the compositions shown in Table VI:

' TA'ELEW 0 Mn Fe s s1 Cu Ni (:1 A1 Ti P MO found to be attributable 16111661656666 of Cr C in item 1 and its substantial absence in items 2 and 3 by Welds were made by TIG welding with the filler materials (wires) shown in Table VIl:

TAELE' v11 c Mn P s s1 c1 Ni Fe Cu Ti A1 M0 N182 .036 3.05 .005 .25 20.12 72.84 .55 .06 .57 0.12 .200 1.90 .014 .012 .41 15.62 34.00 B31. 3 4.5 111601031138 .250 2.00 .025 .75 28.00 38.00 Bill. .05

simiw rspmc observaiiaairie.11611666101601.6565 The weld 1 were him welds y fusion TIG precipitates concentrated at the grain boundaries of specimens of item 1: Ti(C,N) and cr,,c,. In addition, FIG. 11 shows Ti(C,N) dispersed throughout the matrix.

persed throughout the matrix of specimens of item 2, but substantially no Cr C Ti(C,N) is the major precipitate present in item 3 although there are indications of the Cr C, type in an early stage of formation. The Ti(C,N) appears larger than in item 2 and is dispersed throughout the matrix. 1

example 06061621160161 comparison ofincoloy 800 and B-0 Alloy. The compositions of the alloys are shown in the following Table V:

" FIGS. 12'' through 15 show Ti C,N pi'ecipitate diswelding the root pass across the 2 inch face and then filling the joint with filler wire.

Two sections of each weld were mounted for metallographic evaluation. All welds were sound and crack free. Typical photomicrographs are: shown in FIGS. 19 through 27.

Additional samples of each material combination and filler wire were tested by bending around a 1.0 inch diameter mandrel. Results of the bend test are summarized in the following Table Vlll:

TABLE VIII Material Combination Filler Wire Location of Bend Remarks 13-0 alloy to lnco l38-Ni At weld No cracks B-O alloy 82-330 13-0 alloy to lnco 138-Ni On B 0 alloy No cracks 304 SS 82-330 side of weld B-O alloy to lnco l38-Ni On 13-0 alloy No cracks 316 SS 82-330 side of weld H Elevated temperature tensile properties l,l50 F. of welded specimens are given in the following Table IX:

TABLE IXAELEVATED TEMPERATURE TENslLE PROPERTIES 1150 F.

Table 1X Continued WM Total U niform Area Yield Ultimate elong. elong. Failure Material (sq. in.) (p.s.i.) (p.s.i.) (Percent) (percent) site B-316-138 .8994 15,550 56,150 25.0 24.7 Weld B-0-3 16-330 .08865 15,250 48,500 26.7 20.1 13-0 B-0-316-330... .09236 14,900 47,350 18.1 17.3 Weld B-080082.. .09204 13,900 26,200 14.5 11.2 Weld B080082..... .09138 13,150 42,950 22.8 22.0 Weld 8-0-800-138... .09161 13,753 35,500 14.2 13.3 Weld B-0-800-l38... .09178 14,150 45,200 36.8 36.0 Weld 8-0-800-330... .09210 14,000 48,300 38.4 30.9 Weld B-0-800-330 .09186 14,150 54,150 42.0 37.4 B-0 These were made with a typical specimen in the form 15 Z-TIG fillet Welds N1 82 fine! Wire an I the ends of which flare into the web.

EXAMPLE D A tube-to-tube sheet mock-up as shown in FIG. 2 was fabricated to illustrate weldability of B-0 Alloy. The

mock-up consisted of two tube sheets 73, 1 inch thick respectively of 304 stainless steel and B-0 Alloy. Tubes 75 of B-0 Alloy but with high-carbon content were welded to the tube sheets 73 byfusion welding and with filler wire addition.

Chemical compositions of tube sheet alloys and of the tube alloys are given in Table X:

Welds were performed of the high-carbon B-0 Alloy tubes to the tube sheets of 8-0 Alloy and of 304 stainless steel in accordance to the following schedule:

a. l-TIG fusion tre-pan weld trepanning as shown in FIG. 3 No filler addition b. 2-TIG fusion fillet welds No filler addition d. 2-TIG fillet welds 330 filler wire The following results were observed by visual examination:

1. No cracks seen in tre-pan welds of high-carbon B-0 Alloy tubes to 304 stainless steel and 30 Alloy tube sheets. 2. Microcracks were present in TIG fusion fillet welds. 3. Microcracks were present in T16 fillet welds with Ni 82 and 330 filler wire additions.

It is indicated that the microcracking resulted from the high carbon content (0.07) of the high-carbon B-0 Alloy. In addition, the fillet-type weld with or without filler wire imposes high stresses on the weld puddle during cooling, resulting in microcracking and the welding of tubes into tube-sheet without trepanning is notoriously difi'icult.

While preferred embodiments of this invention have been disclosed herein, many modifications thereof are possible. This invention then is not to be restricted except insofar as is necessitated by the spirit of the prior art.

I claim as my invention 1. An iron-nickel-chromium alloy having substantially the following composition in weight percent:

Ni Cu Ti C Mn S Si Cr Fe rapi'aaaein welded structures formed Byar'c welding, wherein the welds are substantially free of cracks, the said alloy being characterized by including sufficient content of said carbon affinity elements to substantially reduce the formation of chromium carbide. 

